The 80-86 pickups all have cable driven speedometers and the speedometer unit comes out with two screws from the back of the cluster. So yes, the medium duty speedometer is a direct bolt in, as long as you get the right year. The very early ones still only had 5 digits plus the 1/10, and at some point they went to an electronic speedometer with a 75mph max.
What I have not seen is a medium duty 6 digit plus 1/10 that also has a trip odometer. If you have a trip odometer now, you would loose that with a medium duty unit.
I believe the wheels can be swapped out, using methods described in the instructions for setting the mileage on a replacement speedometer. The 6 digit plus 1/10 wheels could therefore come from a Canadian Km/Kmh speedometer which are generally easier to find than the medium duty one. I have not done this yet, but intend to try so I can have the additional digit and keep my trip odometer.
The odometer reset instructions are on a tab on this page:
https://www.garysgaragemahal.com/gauges1.html
SHORT BED 4-DOOR DIESEL: 1986 F350 4x4 under construction-- 7.3 IDIT ZF5+GVOD
STRAIGHT SIX 4X4: 1981 F150 2wd to 4x4-- 300 I6 close ratio diesel T19, hydroboost brakes, Saginaw steering
BIG F: 1995 F-Superduty under construction— converting to 6.9L IDI diesel ZF5+DNE2